Precision resistance devices



Jan. 13, 1959 L. E. ASKE 2,868,934

PRECISION RESISTANCE DEVICES Filed April 22, 1954 IN V EN TOR. LEONARDE. ASKE ATTORNEY United PRECISION RESISTANCE DE'VHCES Application April22, 1954, Serial No. 424,934

3 Claims. (Cl. 201-63) This invention in general pertains to an improvedprecision type characterized resistance device and also to a method ofmaking the same.

Characterized resistance devices are broadly old in the art, there beingmany types of these devices as well as several methods of making thesame. A characterized resistance device in general is a resistancedevice wherein a nonlinear change in resistance is effected per unitchange in position of the wiper means cooperating with the resistance.This nonlinearity between resistance change and wiper movement can beachieved by, in one type of device, the use of a resistance windingsupport having a nonlinear cross-section so that successive turns ofresistance wire thereon will have shorter or longer lengths with the endresult being that as the wiper is moved from one turn of wire to theadajacent turns, it successively picks ofi more or less resistancedepending upon the direction of movement, the amount of resistancechange being indirectly dependent upon the change in cross-sectionalarea of the resistance winding support. Another method of obtaining acharacterized resistance device is to have a winding support or bobbinof uniform cross-sectional area such as a cylinder and winding thereupona first section of resistance wire of a certain size for a given numberof turns, then soldering or splicing or by other means securing adifferent size of resistance wire onto the end of the first resistancewire and then winding a certain given number of turns of the secondresistance wire on the support. Additional re sistance sections may beadded with still different sizes of wire as desired until the entiredevice is made.

There are numerous disadvantages to the prior art methods ofmanufacturing characterized resistance devices as well as disadvantagesin the finished resistance devices per se. Disadvantages of thenonlinear Winding support method are that the winding support per se isexpensive to manufacture and elaborate winding machines are required.One disadvantage of the second method is that it is very tedious andtime consuming and delicate inasmuch as each resistance section has tobe Wound, then the winding operation interrupted while a second wire issoldered to the first wire, this type of assembly being time consumingand also requiring very highly trained and skilled workers. Further, itis relatively quite difiicult to solder or splice the adjacent wiresizes together because they are usually very small in diameter anddifiicult to handle. Another problem is the fact that it is difiicult toaccurately control the length of each section of the resistance deviceinasmuch as the soldering often shunts out some of the turns of Wire.Another difiiculty is that the difference in diameter of one section ofthe resistance device as compared to the adjacent sections of resistanceproduces a hump or step therebetween which step is objectionable in manyapplications in that the wiper or slider device cooperating with theresistance device will tend to jump or skip several turns of wire at thestep and hence produce inaccuracies Patented Jan. 13, 1959 "ice in thesystem in which the resistance device is being used. Still anotherdisadvantage of this method is the fact that if any one of theresistance sections becomes damaged requiring its replacement, it isgenerally necessary to discard the entire device resulting in a totalloss. Another difficulty is that Nichrome type wire which has desirableresistance characteristics is generally unsuitable for solderedconnection type resistance devices because it is inherently difiicult tosolder.

It is with the above disadvantage of the prior art resistance devicesand methods of manufacturing in mind that I have devised an improvedprecision characterized resistance device and method of making same. Myprecision characterized resistance device in the preferred embodiment ismade up of a plurality of prewound re sistance sections, each comprisinga washer-like member having metalized portions on each end face thereofand with resistance wire wound on its outer periphery, the ends of thewire being bent over so as to abut against the metalized ends. The sizeof each of the washers would be preselected in accordance to thediameter of wire to be wound thereupon so that the summation of theouter radial dimension of the washer together with the radial dimensionof the wire to be wound thereupon would produce a common dimension.Thus, when a plurality of washers, each of which has wound thereupon aresistance wire of a diameter different from the others, are placed sideby side in axial abutting relationship, their outer peripheral surfacesare all at a common level and there is no hump or step between adjacentresistance sections. Hence, there will be no tendency for a wiper membercooperating therewith to jump or skip several turns of wire as it movesfrom one resistance section to another. Further, there is no need for aseparate soldering operation in joining or electrically connecting oneresistance section to another inasmuch as each washer member has aconductive coating on its end so that when adjacent sections are axiallyabutted against one another, they are electrically connectedautomatically, which connection is maintained throughout the life of thedevice by maintaining an axial pressure upon the whole assembly.

A more complete description of the precision characterized resistancedevice per so as well as the method for making the same will be setforth below but it will be obvious that a broad object of this inventionis to provide an improved precision characterized resistance device andto provide an improved method of making the same.

Another object of the invention is to eliminate the need of wiresplicing, soldering and masking by the coil winding operator in order tocharacterize a resistance device.

A further object of the invention is to accurately control the length ofeach section of a resistance device.

Still another object of the invention is to accurately control theoutside diameter of the bobbin or support member of each wire woundresistance section so that in changing from a small wire size to alarger wire size, the exposed contacting surface of the'entireresistance device will be at a uniform level so as to promote smoothwiper contact.

Another object of this invention is to utilize a type of constructionthat allows the use of resistance wire or" the nickel chromium orNichrome type which can be used due to the elimination of solderingoperations.

A further object of the invention is to eliminate the use of windingsfor a dead spot on a resistance device and thus provide for moreaccurate control of the dead spot length.

A further object of the invention is to provide a method ofmanufacturing precision characterized resistance devices whereby greatlatitude in design is permissive due to the universal sectional assemblymethod.

Still another object of the invention is to provide a resistance devicewherein a damaged resistance section may be replaced without discardingthe other resistance sections.

Another object of the invention is to provide a characterized resistancedevice wherein machine winding technique may be utilized to the fullestso that uneven tension on the wire produced by hand winding operation iseliminated as well as finer control and spacing of the turns of wire oneach resistance section.

The above and still further objects and advantages of the invention willbe made more clear in the following specification and drawings in which:

Figure l is a side elevational view of a potentiometer device utilizingin part an improved precision characterized resistance member;

Figure 2 is a cross-section view of the resistance member shown inFigure 1 taken along section line 22;

Figure 3 is a side view of an individual winding section or bobbinmember;

Figure 4 is an end View of the bobbin shown in Figure 3;

Figure 5 is an illustration of a plurality of winding sections orbobbins and spaces between winding sections prior to the windingoperation;

Figure 6 is a showing of the same assembly shown in Figure 5 with theassembled winding sections or bobbins and spacers being wire covered;

Figure 7 is a side view of an individual resistance section after it iswire wound and before the ends of the wire are bent over;

Figure 8 is an axial end view of the individual resistance section shownin Figure 7 with the ends of the wire bent over so as to abut againstthe conductive film which is disposed on the axial end of the disc; and

Figure 9 is a cross-section view of a modified resist ance device.

In Figure l is shown a typical application of the improved precisioncharacterized resistance device where in a characterized resistor it) issupported by a screw member 12 which has a headed portion 14- extendingup through an aperture 16 in an upper support plate A curved snap washer2th positioned between the top of support plate 18 and the underside ofhead member 14 on screw 12 holds a shoulder portion 22 of screw 12 upagainst the underside of support plate 13 but allows rotation of screw12 with respect to support plate 18.

A lower support plate 24 having a boss member as with a conical recess28 therein provides a bearing support for the lower cone-shaped end 39of screw member Also mounted on lower support plate 24 is a pressureresponsive bellows member 32 and a bracket 3%, the latter of whichserves as the pivotal support for a wiper carrying arm as. Bellowsmember 32 is operatively connected by means of a link 38 to anintermediate point of arm 36, which latter member carries on itsunpivoted extremity 4d a pair of wiper members 42 which are adapted tobear against the resistance member 19.

Referring to Figures l and 2, it is seen that resistance device lbcomprises a bobbin or support member which may be fashioned out of anysuitable rigid material, metallic or nonmetallic, such as aluminum orBakelite. Support member 44 has a lower headed portion 46 as well as aninner bore 48, the lower portion of which is threaded as at 5% so as tocooperate with the threaded portion on screw member 12.

A pin member 52 is secured in a suitable aperture in the headed portionof support member and extends parallel to the longitudinal axis ofsupport member 44. A hole 54 in the lower support plate 24- serves as aguideway for guide pin 52. Thus, when screw member 12 is rotated withrespect to support member 13,

rotation of resistance member 10 is precluded by guidc pin 52 beingengaged in hole 54 and hence the elevation of resistance member 1dvaries in accordance to rotation of so: v. member 12. Resistance member10 may thus be adjusted with respect to wipers 4-2.

Positioned intermediate on support member 4 iare a plurality ofresistance sections 56, 6d and 62. Each of the resistance sectionscomprises a washer-like member 6 shown in greater detail in Figures 3and 4, said washers being made [1"..21 of some nonconductive materialsuch as p astic or out of a conductive material with an in re exterior.Cine material that has proved very satisractory for this member is Pyrexglass. Each washer member as has an outer periphery 66 and an inner boreof bore 68 being such that the inwill fit snugly on support member .4.

In the preferred embodiment shown, the resistance sections comprise awasher-like member 64 as a winding form. However, it is to be wellunderstood that the teaching of the invention may be applied to othershapes of winding forms and thus the invention should not be limitedonly to washer-like elements. A further point to be kept in mind is thatwhile the abutting surfaces of the adjacent resistance sections areshown to be normal to the longitudinal axis of the device, otherarrangements will occur to those skilled in the art and the inventionshould be so construed.

Prior to the winding operation, each washer 64 has a metalized portionor conductive film 7t} placed on each end face thereof. Variousmetalizing procedures may be used to place the conductive film '70 onthe end faces of discs 64*. One metalizing procedure that has been foundto be successful is an embossed gold leaf transfer process. However,other metalizing processes may be used and the invention should not belimited to any one specific type of procedure. While conductive film 7i)is shown on Figure 3 to have a considerable axial extent, it is inpractice only of miscroscopic thickness, the thickness in the drawingbeing exaggerated in order that its presence be noted.

it is to be noted that the conductive film 70 on the end faces of washermembers 64 does not extend all the way to the inner periphery 68 of thewashers. The reason for this is that it support member 44 is metallic orelectrically conductive, it will not short out adjacent resistancesections. Obviously, if support member 4-4 were nonconductive, thenconductive film '70 could cover the entire end face of each washer 64.

in the preliminary design of a resistance device, it will be calculatedas to what axial length and what wire size is desired for eachresistance section. With this preliminary data in mind, the axiallengths of each washer member 6 is determined as well as the outerdiameter 66 of each washer. That is, the axial length of each washer 64should coincide with the desired axial length of the particularresistance section. Further, the outer diameter 66 of the specificwasher 64 should be such that when it is covered with successive turnsof wire, the outer surface of the wire should be at a common level awayfrom the center of the bobbin 01' support member 44. To explain further,if a resistance wire of relatively large dimension is to be used in aparticular resistance section, the outer diameter 66 of that particularwasher member 64 should be proportionately smaller as compared to anadjacent I resistance section which, for this illustration, would have aresistance wire thereon of much smaller diameter. Thus, by having theouter diameter of the individual washer members 64 inverselyproportioned in accordance to the size of wire thereupon, the outersurface of the wound sections is at a common level and no steps or humpsare produced between adjacent resistance sections and consequently asmooth surface will be presented to the wiper members 432.

After the individual washer members 64 have their conductive film 7dthereon, all of the washers 64 which have a common outer diameter andwhich are to receive the same size resistance wire thereupon are placedon a mandrel member 72 shown in Figures 5 and 6. Mandrel member 72 has aheaded portion 74 at one end thereof and on the opposite end has threads76. A stripper washer 78 is first placed on the mandrel member 72 andabuts against headed portion 74. Then the individual washer members 64are placed on the mandrel, being interspaced by additional washermembers or spacers 88. These spacer washers 80 are preferably made outof a material commercially available under the trade-name Teflon. Whilewasher members 80 may be of any suitable material, Teflon is one thathas been found to be very satisfactory. Teflon is made by the E. l. duPont Company and is a polytetrafluorethylene. Another fiuorinatedhydrocarbon material which may be used is known as fluorothene or Kel-F,a trifluoromonochloroethylene substance made by the M. W. KelloggCompany which has characteristics generally similar to those of theaforementioned Teflon. The fluorinated hydrocarbon materials abovesuggested are most desirable for this particular purpose because of aninherent slipperiness and also the additional characteristic which isthat most adhesives such as glue and varnish will not adhere to it.Obviously, other materials having the properties mentioned to the extentnecessary for the particular application in mind may be used.

After the washer members 64 and spacer washer members 80 are placed onmandrel member 72 in interspersed relationship, the entire assembly isclamped together by a nut member 82 screwed down over threads 76 on theone end of mandrel member 72. It is to be noted that the outer diameterof the spacer washer members 80 is the same as the outer diameter ofwasher members 64.

The assembled washers 64 with spacer washers 8d are then given a coatingof varnish and allowed to dry somewhat so that the varnish becomestacky. Obviously, other suitable adhesives may be used in place ofvarnish for this purpose. I have found that a convenient method ofgiving the assembled washers a coating of varnish is to dip theassembled sections in a vat of varnish having a viscosity of 60 to 70centipoiseswith a drying time in air from 8 to 24- hours. I have foundthatwithdrawing the mandrel from the varnish vat at a rate of 2 inchesper minute willgive a desirable coat of varnish of a thickness from.0004 to .0005 inch.

Due to the special slippery properties of the Teflon washers 80, thevarnish does not adhere thereto but only to the exposed peripheralsurfaces 66 of the washers 64.

After the dipped assembly of washers has dried a said cient length oftime, the mandrel is placed on a winding machine and an enamel orotherwise insulated wire of the desired size and resistivity is woundthereon using a conventional machine winding process producing afinished result as shown in Figure 6. It will be noted that the windingmachine winds over the spacer washers 80 as well as the washer 64.During the winding process, the wire is embedded into the thin coatingof varnish on each of the washers 64 but naturally does not adhere tothe washers 80 inasmuch as they did not have a coating of varnishthereupon. Thus, after the winding operation, each individual washermember 64 has its resistance wire attached thereto. The bonding of thewire to the washer 64 is further improved by bakingthe entire mandrel assembly which. accelerates the drying of. the varnish. I have found thatbaking the assemblies at a temperature of 265 F. for a period of time oftwo hours is satisfactory.

During the baking process, the wire is bonded to the peripheral surfaceof the washer 64 from the underside of the wire. Sufficient adhesion isacquired during the baking process so as to make it unnecessary to coatthe outer surface of the wire with a special adhesive as is sometimesdone in winding conventional resistance devices.

After the baking process, the individual wire wound Washers 64 may beremoved from the mandrel, this being done by removing nut 82 andexerting axial pressure against the side of stripper washer 78 whichforces washers 64 and 80 off the threaded end of mandrel 72. Theindividual resistance sections are separated from the Teflon spacerwashers 80 by clipping off and discarding the excess coiled wire Woundon the Teflon sections. Inasmuch as the axial length of the Teflonwashers 88 is relatively short with respect to the axial length of thewashers 64, very little resistance wire is wasted. The wire should becut so that short unbonded wire ends 84 and 84 are left on each end of aresistance section as shown in Figure 7. The next step is to remove theenamel or other insulative coating from the free ends 84 and 84 and thenbend ends 84 and 84 in against the such as du Ponts #4922 conductivesilver coating may be employed; a small amount of the coating beingsufficient to hold ends 84 and 84 in place.

It will be understood that each of the resistance sec tions 56, 58, 60and 62 assembled on support member 44 are independently wound in thepreliminary winding process. That is, all of the resistance sections 56for a plurality of resistance devices would be wound at the same timeand so on for the resistance sections 58, 60 and 62.

In the actual assembling of the resistance devices, an insulative spacermember 88 having a headed portion 90 is first slipped down over theshank of support member 44 until the headed surface 90 abuts against theheaded portion 46 on support member 44. The axial portion of insulativespacer member 88 serves to space a metallic spacer member 92 away fromsupport member M:- as is clearly shown in Figure 2. Positioned betweenthe bottom axial end of metal spacer member 92 and headed portion 90 ofthe insulative spacer 88 is a conductive terminal member 94 having asuitable tab 95 thereon for connecting external circuit means (notshown) to one end of the resistance device. Next the plurality ofresistance sections 62, 60, 58, and 56 are slipped over the shank ofbobbin member 44 and are placed in axial abutting relationship with oneanother. Each of the resistance sections is carefully oriented so thatthe ends 34 and 8 5 on adjacent resistance sections do not abut againstone another. Thus the adjacent resistance sections are electricallyconnected to one another inasmuch as their conductive portions 70 withthe ends 84 and 84 are touching one another with the only gap betweenadjacent sections being equal to the diameter of the largest wirepresent. Further, they are electrically connected without the necessityof any separate soldering operation. An additional metal spacer member96 is abutted against the top portion of resistance section 56 and iselectrically insulated from bobbin member 44 by another insulativespacer member 98 which may be identical to spacer member 88, the formeralso having a headed portion 100 thereon. Another terminal member 102 ispositioned between the top end of metal spacer 96 and the headed portion100 on insulative spacer 98 and serves to connect the other end of theresistance device to external circuit means, not shown. A washer member104 abuts against the outboard axial end of insulative spacer member 98and a curved snap ring 106 fitting in a notch 108 on bobbin member 44serves to maintain the entire assembly in a tight relationship whichassures electrical continuity from terminal member 102 through metallicspacer member 96, resistive portions 56, 58, 60 and 62, metallic spacer92, and to terminal member 94.

If it is desired to have a tapped resistive device then an additionalterminal member 110 similar to terminal members 102 and 94 may bedisposed between adjacent 7, resistance sections such as is shown inFigure 2 between resistance sections 56 and 58. This, of course, wouldbe an optional feature depending upon the application desired.

Another optional feature of this invention is shown in Figure 9 whereina modified construction of a resistance device is disclosed generallysimilar to- Figure 2 but having a deadspot between two resistancesections 60 and 62'. T he deadspot is formed by a conductive spacer 1%generally similar to spacer members 92 and 96 and p0 tioned with respectto support member 3 by a sleeve member 122 of insulative material. Usingthis method of construction a deadspot between adjacent resistancesections is provided, the axial extent of the deadspot, of course, beingequal to the axial length of the spacer rncmher 129. I have found thisto be a much simpler and more accurate method of providing deadspots ina resistance device than prior art methods such as shorting out aplurality of turns of the resistance wire with a conductive coating suchas solder or the like.

After the resistance device has been assembled and secured in place bybowed snap ring 196, an axial portion of the winding is exposed by ahurling operation as at 112 shown in Figure l. This butting operationremoves the enamel or other similar insulation on the exposed peripheryof the resistance wire and thus allows wiper members to make electricalcontact with the individ ual turns of wire.

It will be appreciated that displacement of the bellows member or anyother type of device used to actuate wipers 4-2 will result in rotationof arm member 36 about its pivot point so as to vary the verticalelevation of wiper members 4-2. Wiper members 42 would obviously beconnected to external circuit means, not shown, so that changes inelectrical resistance in accordance to displacement of the actuatingdevice could be used for some useful purpose.

While 1 have shown and described a specific embodiment of thisinvention, further modifications and improvements will occur to thoseskilled in the art. I desire it to be understood therefore, that thisinvention is not limited to the particular form shown and I intend inthe appended claims to cover all modifications which do not depart fromthe spirit and scope of this invention.

What I claim is:

1. A precision resistance device of the class described comprising asupport, a plurality of insulative washers mounted coaxially on saidsupport so that the end faces of adjacent washers abut one another, aconductive film on said end faces of said washers, resistance wire woundabout substantially the entire outer periphery of said washers, the sumof the outer radius of one of said washers, plus the radial thickness ofthe resistance Wire wound on said one of said washers being equal to therespective sums of the outer radius of the other of said washers plusthe radial thickness of the resistance Wire wound on said other of saidwashers, said resistance wire having ends abutting said conductive filmson said end faces, metal spacer means abutting the end faces of saidplurality of discs, terminal means abutting said spacer means, meansinsulating said spacer means and said terminal means from said support,and means holding said washers, spacer means, and terminal means tightlytogether.

2. A precision resistance device of the class described comprising asupport, a plurality of forms with faces thereon mounted on said supportso that said faces of adjacent forms abut one another, conductive meanson said faces of said forms, resistance means found on substantially theentire periphery of said forms, the sum of the cross-sectional dimensionof one of said forms plus the thickness of the resistance means wound onsaid one of said forms being equal to the respective sums of thecross-sectional dimension of the other of said forms plus the radialthickness of the resistance means wound on said other of said forms,means connecting said resistance means to said conductive means on saidfaces, terminal means contacting said plurality of forms, and meansholding said forms and said terminal means in assembled relationship.

3. A precision resistance device of the class described comprising asupport, a plurality of insulative washers mounted coaxially on saidsupport so that the end faces of adjacent washers abut one another, aconductive film on said end faces of said Washers, resistance wirehaving different diameters wound about substantially the entire outerperiphery of said washers, the sum of the outer radius of one of saidWashers, plus the radial thickness of the resistance wire wound on saidone of said washers being equal to the respective sums of the outerradius of the other of said washers plus the radial thickness of theresistance wire wound on said other of said washers, said resistancewire having ends abutting said conductive films on said end faces, metalspacer means abutting the end faces of said plurality of discs,terminal. means abutting said spacer means, means insulating said spacermeans and said terminal means from said support, and means holding saidWashers, spacer means, and terminal means tightly together.

References fired in the file of this patent UNITED STAi ES PATENTS

